That is true, but JSON is typically serialised as JSON anyway so that is not the issue.
It's most likely the the format of the JSON and potentially also the setup of your webhook.
Can you also show the setup of your webhook?
BTW, have you seen this workshop
It also addresses Azure webhook setup.
Also, have you searched the forum for "azure json" and looked at the proposed solutions?
It's always good to have a thorough browse through the forum threads that touch on your issues at hand.